1. Field of the Invention
This invention relates generally to semiconductor processing, and more particularly to semiconductor chip solder bump structures and methods of making the same.
2. Description of the Related Art
Flip-chip mounting schemes have been used to mount semiconductor chips to circuit boards, such as semiconductor chip package substrates. In many conventional flip-chip variants, a plurality of solder joints are established between input/output (I/O) sites of a semiconductor chip and corresponding I/O sites of a circuit board. In one conventional process, a solder bump is metallurgically bonded to a given I/O site or pad of the semiconductor chip and a so-called pre-solder is metallurgically bonded to a corresponding I/O site of the circuit board. Thereafter the solder bump and the pre-solder are brought into proximity and subjected to a heating process that reflows one or both of the solder bump and the pre-solder to establish the requisite solder joint.
In one conventional process, solder bumps are fabricated on a semiconductor in a printing process. A stencil or mask with suitable openings leading to underlying conductor pads is formed. The openings are next filled with a solder paste and the stencil is then removed. A reflow is then performed to convert the solder paste into a solder bump. In another conventional process, solder bumps are fabricated in a plating process. A stencil or mask with suitable openings leading to underlying conductor pads is formed. Using a metal film, typically some blanket-deposited film, on the semiconductor chip as a plating bar, the openings are next filled by plating a solder mixture. The stencil is removed and the plated solder is reflowed to round out the bump.
A difficulty associated with conventional solder bump plating processes is composition control. For example, plating a tin-copper solder involves plating a pre-mixed solution of copper and tin. The solution will typically have a particular ratio of copper to tin. However, during the plating process, the mixture of the tin and copper that actually adheres to the plating bar may vary from the desired composition. While such deviated bumps will still conduct current, another issue may arise. The melting temperature of some types of solders are quite sensitive to solder composition. For example, some types of tin-copper solder can exhibit melting temperatures that significantly approach or exceed desired reflow temperatures if the copper content varies by 1% or so from the desired composition ratio. Structures in and around the solder bumps, such as a circuit board or components thereof, may not be able to withstand the unexpectedly high temperatures necessary to reflow such deviated bumps, and the solder may not even liquify.
The present invention is directed to overcoming or reducing the effects of one or more of the foregoing disadvantages.